1. Field of the Invention
The present invention is related generally to audio amplifiers, and more specifically to an amplifier that automatically enhances the efficiency and quality of the response of the output signal in various audio frequency ranges that are audible to the human ear.
2. Description of Related Art
It is well known that to the human ear the amount of distortion that is discernable is different for different audio frequency ranges. At low frequencies a much greater amount of distortion must be present before a listener can hear it as compared to the amount of distortion that can be discerned at frequencies in the 1000 Hz range. Additionally, low frequency speakers are slow to respond to music due to the mechanical design, size and weight of the component parts of the speaker thus distortion is often rendered inaudible (i.e., low frequency speakers are physically large in size). This fact, coupled with the characteristics of the human ear at low frequencies, permits the audio signal to include a much higher percentage of distortion before it can be heard by a listener. Thus at lower frequencies, efficiency of the audio system is more important than the quality of the sound produced.
However, at higher frequencies, where the human ear is better able to distinguish between sounds, speakers designed to reproduce sound in that frequency range are also much more responsive to the audio signal applied to them due in large part to the light weight of the component parts of the speaker (i.e., the higher the frequency the speaker is physically small in size). This improved speaker response to the applied signal results in the speaker being much less likely to mask distortions in the audio signal than their low frequency speaker counterparts. These two factors, human ear and speakers each being more responsive to higher frequencies in the 1000 Hz range creates a challenge to the designer to minimize distortion in the audio signal delivered to the higher frequency speakers. Therefore in this frequency range the quality of the sound produced is of critical importance.
Since the moving mass (i.e., speaker cone and coil) is much smaller in the range of 1000 Hz than in low frequency speakers, the higher range speakers almost instantaneously respond to small amplitude changes in the applied signal. These smaller amplitude changes are often not a part of the desired audio information (e.g., music). For example those small signal variations could be modulation noise in the signal or on the supply rail resulting from an amplifier, such as that of Hamada (U.S. Pat. No. 4,054,843), is known to have a lot of problems introduced by the modulation technique that is used. The Hamada amplifier basically tracks 100% of the audio signal at its feed. The Hamada amplifier simply cuts the voltage to a exact level that is dictated by the audio signal, regardless of the frequency of the signal and the frequency response of the speakers and the human ear. Therefore, at higher frequencies the Hamada amplifier becomes problematic for several reasons, for example, it inhibits and delays, to some extent, to prepare the amount of energy needed for the amplification when that energy is needed to be supplied on time and equal to the level needed to amplify the signal. This makes the task of the Hamada amplifier difficult and unachievable and compromising when it comes to distortion. Adding consideration of the sensitivity and frequency response of the human ear, the problems at the mid-range and higher audio frequencies are accentuated. Thus at these higher frequencies, more quality is needed in the sound amplification than efficiency.
Given the sensitivity of the human ear, much more amplification is needed below 100 Hz and less amplification at 1000 Hz, and depending on the listeners hearing response more or less amplification at 5000 Hz since the upper end sensitivity of the human ear varies dramatically from person to person. Thus, in the sub harmonic or the sub woofer range the ear needs more amplification and distortion is not audible as discussed previously, however any sensitive change of the amplifier at 1000 Hz is going to be clearly audible and compromise the sound reproduction quality of the amplifier. By tracking the audio signal exactly for all frequencies, as with the Hamada amplifier, and providing a fixed headroom for the signal, regardless of the frequency of the signal, the quality, or lack thereof, of sound reproduction is very evident.
What is needed is an amplifier that takes into consideration the differences in performance of speakers at different frequencies, as well as the natural variations in the frequency response of the human ear. The amplifier of the present invention provides such a doubly, automatically compensated amplifier.
The present invention provides several different embodiments each achieving the same results and improved performance over the prior art. The amplifier of the present invention causes variations in the rail voltage that are controlled by the frequency of the audio signal being amplified as the audio signal changes. This is done to tune the operation of the amplifier taking into consideration the frequency response of the human ear. By doing so, the resulting amplifier is more efficient at lower frequencies where more signal distortion is required before it can be heard by the human ear, and a higher quality signal output is provided at higher frequencies where the human ear can more readily detect distortion. Thus by designing the amplifier from the listener""s point of view results in an amplifier with much improved performance from both technical and listener points of view.